The objective of the proposed project is to perfect a miniature implantable cardiac pacemaker powered by the mechanical forces of ventricular contraction through the medium of a piezoelectric generator and electronic circuitry. Acute and chronic experiments in dogs and marine mammals have demonstrated the adequate functioning of such a device. Encapsulation has been a major problem but a satisfactory solution appears now to have been found. Long-term testing of the electronic components of the pacemaker and of their encapsulation needs to be pursued further. Dogs and chimpanzees will be used in the continuing in vivo studies, with the eventual aim of developing a batteryless implantable pacemaker for use in human subjects.